1. Field of the Invention
The present invention relates to recordable optical media, such as discs and cards, of the WORM (write once, read many) type with fluorescent data reading, and in particular to the use of such media in a high-capacity 3D optical memory.
2. Description of the Prior Art
The memory volume and signal-to-noise ratio of current optical media are insufficient for new-generation computers and videosystems. Therefore, intensive development of materials for improved optical storage systems is in progress, with the goals of enabling a higher recording density, high signal-to-noise ratio, high resolution and low cost.
Recently, significant attention has been drawn to optical media of the WORM type in which information can be written in a form allowing its retrieval immediately following the recording. The capability of real-time recording is essential for using optical recording in various devices, in particular and primarily for computer systems. The optical recording principle could enable not just unitary copies of data carriers but replication of data in relatively small volumes which today is increasingly in demand.
The underlying principle for all optical recordable materials of practical interest is the photothermal recording. Data are recorded on such materials by scanning the recording layer by a focused laser beam. The energy of the laser beam is absorbed by the layer""s active medium and turns to thermal energy causing its physical and chemical changes that can be optically recorded while reading. Among diverse types of photothermal recording on WORM-type optical materials, ablative recording has been chosen for predominant practical application. The characteristic feature of ablative recording is that at melting, evaporation or chemical transformation of the active layer there occur geometric changes of the layer which can be optically registered owing to the change in the reflection factor of the active medium.
Among the materials for ablative recording, considerable attention is drawn to optical WORM discs using as a recordable medium thin (10 to 100 nm) layers of organic dyes of both binder-free and dye-in-polymer types. The layers of organic dyes possess a number of significant advantages as compared to, for instance, metal and semi-metal layers used in WORM discs with ablative recording. The advantages are as follows:
Dyes can have a more selective absorption at the recording laser wavelength.
Dye layers possess a higher stability at higher humidity.
Dye layers are more sensitive to laser radiation due to their low thermal conductivity, thermal capacity and low melting or decomposition point. In this connection, they ensure a higher writing density.
Dye-layers-based materials enable a higher signal-to-noise ratio, as the amorphous layers"" own noise is negligible.
Dye layers are made by a simple and cheap spin-coating procedure rather than by vacuum deposition technique applied in manufacturing WORM discs with metal or semi-metal layers.
The capacity of currently manufactured WORM discs based on organic dyes is as high as 4.7 Gigabytes. Such capacity is attained at the expense of higher writing density of the DVD format as compared to the CD. The capacity can be further made twice as high through using two layers. Yet, a further increase in the number of active layers in a WORM disc with ablative recording and reading by changes in the reflection factor leads to a dramatic deterioration of data reproduction due to diffraction of the recording and reading laser beams in ablation-caused irregularities resulting in failure to enlarge the disc capacity. The promising way to increase the capacity of the optical memory carrier is to obtain a multilayer disc with fluorescent reading.
A WORM disc with fluorescent reading has been proposed. The principle of this technique is in that following the process of writing, the information recording centers do not fluoresce, while the background does. At reading by an appropriate laser beam, fluorescent light is excited and recorded by a detector.
The optical recording disc is arranged in such a way that in it, the active layer is applied onto a matte surface of the substrate. Hence, no multilayer discs could be created based on the discs because in them the recording and reading laser beams would undergo considerable scattering.
The most frequently proposed materials to obtain active layers in WORM discs are cyanine dyes, phthalocyanines and porphyrins. Many dyes from these classes possess satisfactory fluorescence in polymer matrices when they form actual solutions, i.e. when they are in a polymer in a molecularly dispersed state. In existing WORM discs with ablative recording, dyes are applied onto a replica with a spiral track as binder-free amorphous thin layers in which the dyes used do not fluoresce.
In some cases, to obtain active layers in WORM discs, dyes-in-polymers are used. To achieve the appropriate level of sensitivity of active layers enabling ablative recording, a maximum high concentration of dye in a polymer matrix is provided, for which due to concentration quenching fluorescence is either drastically reduced or absent altogether.
Consequently, known types of materials applied in single-layer WORM discs and techniques of photothermal data recording thereon cannot be used for multilayer optical recording media with fluorescent reading.
The present invention has been made in view of the above circumstances and has an object to provide a multilayer optical recordable medium with fluorescent reading enabling a high-capacity optical memory, high signal-to-noise ratio and high sensitivity to recording laser radiation, which may be in any of the ultra-violet, visible and near infra-red ranges.
Further, the present invention provides a multilayer optical recordable medium with fluorescent reading ensuring high-speed and high-density photothermal recording.
Further, the present invention provides a multilayer optical recordable medium with fluorescent reading enabling a high storing stability of the material prior to and following the recording and while reading information.
In compliance with the purpose of the present invention, the above-mentioned multilayer optical recordable medium with fluorescent reading represents a disc or a card, in which each active layer is applied onto a transparent film substrate and includes at least two phases, one of which (fluorescing) contains at least one luminophore capable of absorbing reading radiation of the laser and emitting fluorescent light, while the other phase (quenching) contains at least one substance capable of quenching the luminophore""s fluorescence (quencher).
Further, in compliance with the present invention, data recording on the optical medium is provided as a result of irreversible incidence of a fluorescent signal occurring at luminophore-quencher interaction under the action of focused laser radiation that causes heating of the medium. In so doing, prior to the recording the active layer fluoresces, while following the recording the fluorescence disappears in those places that have been affected by the laser light. Data writing and reading can be done by laser radiation of both identical wavelength and different ones. In the former case, data are recorded and read by different laser radiation powers.
Furthermore, in compliance with the present invention, the fluorescing and quenching phases represent polymer thermoplastic layers in immediate contact with each other, or the fluorescing or quenching phase represents two polymer thermoplastic semilayers between which there is a quenching or fluorescing phase, respectively, serving as a polymer thermoplastic layer, or the fluorescing phase represents a polymer thermoplastic layer while the quenching phase represents a transparent binder-free quenching layer, or the fluorescing phase represents two polymer thermoplastic semilayers between which there is a quenching phase as a transparent layer of polymer binder-free quencher. There could be a spacing layer between the fluorescing and quenching layers.
Further, in compliance with the present invention, the fluorescing phase represents a polymer thermoplastic layer comprising a fine-grained sorbent with a luminophore absorbed on its surface, or the quenching phase represents a polymer thermoplastic layer containing a fine-grained sorbent with a quencher adsorbed on its surface, or the two-phase system represents a polymer layer of fluorescing phase with distributed dispersion of the fluorescing phase, or the two-phase system represents a polymer thermoplastic layer in which dispersions of the fluorescing and quenching phases are distributed. Dispersions of the fluorescing and quenching phases could represent small particles of a luminophore or a quencher, respectively, or a fine-grained sorbent with a luminophore or a quencher adsorbed on its surface, or finely divided solid solution of a luminophore or a quencher in a polymer binder.
Furthermore, in compliance with the present invention, to ensure requisite sensitivity of the active layer while using a laser with identical wavelength for both reading and writing, a luminophore and/or quencher are used absorbing radiation of the recording laser; in addition, to the fluorescing and/or quenching phase and/or a spacing laser between them is added a non-fluorescing dye capable of absorbing the laser""s recording radiation and convert the absorbed light energy to heat (light absorber).
When a light absorber is introduced into the active medium, there can be used a luminophore and/or a quencher incapable of absorbing at the wavelength of the recording laser. In this case, reading and writing are done by lasers of different wavelengths.
The object of the present inventionxe2x80x94to provide an optical WORM-type storage enabling considerably better characteritics of optical memory capacity, signal-to-noise ratio and sensitivity to the recording laser radiation as compared to ones for existing optical recording mediaxe2x80x94is based on two fundamental principles: 3D (multilayer) memory and fluorescent reading. To ensure high writing rate and density as well as high storing stability of the material prior to and following the recording and while reading information, the present invention makes use of the widely applied and well-proved principle of photothermal recording. However, in contrast to existing WORM discs for ablative recording causing burning of holes, generation of bubbles and changes in the surface texturexe2x80x94all of which, as mentioned above, are absolutely unacceptable for a multilayer material due to diffraction of the laser beamxe2x80x94the present invention provides a photothermal recording technique leading to no geometric changes in the active layer. This is achieved thanks to the fact that the active medium consists of two phases, one of which (fluorescing) comprises a luminophore while the other (quenching) contains a fluorescence quencher. Prior to writing, the active layer fluoresces, since the luminophore and the quencher are in different phases, and there is thus no interaction causing fluorescence quenching. During writing, the focused laser beam energy is absorbed by the active medium and converted to thermal energy. As a result of local heating, the quencher diffuses into the fluorescing phase and reacts with the luminophore, which leads to fluorescence quenching. In this way information is recorded without deformation of the active layer. Data reading is done by scanning the layer by a laser beam exciting fluorescence and by detecting differences in the fluorescent signal intensity between the background and writing spots. Data recording and reading can be done by laser radiation of both the same and different wavelengths. In the former case, reading and writing are done by different laser power radiations.
As pointed out above, the important purpose of the present invention is to provide a multilayer optical recordable medium with fluorescent reading that would ensure high sensitivity to the recording laser radiation and hence high-speed recording and higher information density.
It is known that the sensitivity of optical recordable materials based on the principle of photothermal recording is determined by two major factors: the capability of the active medium to absorb laser radiation and the mass of the material being heated. As the optical density of the medium increases and the mass of the medium decreases, the sensitivity of the material increases. To ensure adiabatic conditions of heating, short writing pulses are used, with low-thermal-conductivity materials chosen for the active medium and matrix. In compliance with the present invention, the crucial sensitivity-defining criteria for the recording optical disc is the effectiveness of the process of approaching of the luminophore and the quencher in the heated spot to a maximum distance of 50 Angstroms to enable fluorescence quenching by the Foerster resonance mechanism.
In the original disc, the fluorescing and quenching phases containing a luminophore and a quencher, respectively, should have such structure and composition and be disposed relative to each other in such a way that a requisite sensitivity is ensured at writing, namely: they should have optimal parameters of optical density, mass and thermal conductivity of the heated material and a high efficiency of fluorescence quenching. The conditions are met in the present invention at the expense of a number of factors, the most important ones being
the fluorescing and quenching phases are disposed as will be disclosed below;
the luminophore and/or quencher absorb the recording laser radiation and convert the absorbed light energy to heat;
the fluorescing and/or quenching phase(s) contain(s) a light absorberxe2x80x94a non-fluorescing dye capable of absorbing the laser recording radiation and converting the absorbed light energy to heat;
the quencher absorbs light in the spectral region of the luminophore fluorescence;
the applied polymer media possess requisite thermoplastic characteristics enabling high-speed convergence of the luminophore and quencher in the heated spot; the polymer media are doped with plasticizers to ensure a higher speed of luminophore-quencher convergence in the heated spot.